Vacuum oven muffle seal



Dem 1964 F. A. POTENZO ETAL 3,160,404

VACUUM OVEN MUFFLE SEAL Filed Feb. 17, 1961 \36 INVENTORS FRANK A. POTENZO -le BY RAYMOND L. SHEARER ATTORNEYS United States Patent Ofifice Eif fififid Patented Dec. 8, 1964 3,160,404 VACUUM OVEN MUFFLE SEAL Frank A. Potenzo and Raymond L. Shearer, Kennebunk,

Maine, assignors to The Baker Company, Inc hiddeford, Maine, a corporation of Maine Filed Feb. 17, 19M, Ser. No. 90,096 4 Claims. (Cl. 26341) This invention relates to industrial ovens, or more particularly, to a novel improvement in what are generally referred to as vacuum ovens.

In certain fields, and particularly the field of semiconductor manufacture, there has developed a need for relatively small ovens which can be arranged in conjunction with a line of sealed work spaces which are referred to as dry boxes. A controlled environment is maintained in the dry boxes. Gas lock installations between the dry boxes allow the parts to be transferred from one station to another without danger of transferring contaminants. The dry boxes are fitted with hand holes which accommodate surgical rubber gloves or the like whereby a workman can handle parts within the dry box from externally of the dry box without destroying the integrity of the box. Vacuum ovens may be placed in the line of dry boxes so that work can enter an oven from one dry box and be removed from the oven into another dry box. In some instances, a vacuum oven may be mounted at the end of a line so that parts may be placed in the oven through one door therein and then removed from another door communicating with one of the dry boxes. The vacuum oven derives its names from the fact that the oven is evacuated to a relatively low pressure during the heat treatment of the parts in the oven. The ovens are usually fitted with shelves arranged vertically in the oven and on which parts may be placed for heat treatment.

One of the problems which has arisen in connection with vacuum ovens is that of heat distribution in the oven. For example, in the manufacture of transistors, it is important that the temperature at which the parts are treated is maintained very closely. With many small parts spread over a plurality of vertically arranged shelves, it is important that the heat distribution within the oven, both in a vertical and in a horizontal direction, be substantially uniform so as to assure that each of the parts is exposed to the some desired temperature. Another problem which has arisen in connection with the vacuum oven is concerned with the sealing around the doors of the oven. Because it is more resistant to high temperature, silicon rubber is a preferred material for use as a door seal. However, when the silicon rubber is exposed to high temperature and at the same time, is exposed to very low pressures, the volatiles in the silicon rubber tend to boil off readily. These volatiles may contaminate the parts in the oven. Further, the rubber quickly becomes hard and brittle and deteriorates into the condition where it no longer performs the desired function.

It is accordingly an object of this invention to provide a vacuum oven of the type described which will provide a more uniform heat distribution within the oven.

It is a further object of this invention to provide an improved door sealing arrangement for a vacuum oven of the type described whereby the service life of the sealing material, particularly silicon rubber, will be materially lengthened.

Other objects will be in part obvious, and in part pointed out more in detail hereinafter.

The invention accordingly consists in the features of construction, combination of elements and arrangement of parts which will be exemplified in the construction hereafter set forth and the scope of the application of which will be indicated in the appended claims.

In the drawings:

FIG. 1 is a cross sectional view of a vacuum oven 1ncorporating the present invention; and

FIG. 2 is an enlarged fragmentary cross sectional view of the portion of the oven of FIG. 1 enclosed by broken lines 2.

With reference to the drawings, a vacuum oven of a type with which this invention is concerned generally comprises a stainless steel mufiie 10 which, in the specific embodiment shown, is an enlongated hollow member having a generally rectangular cross section and being open at its opposite ends. The muflle is contained within a housing which comprises a pair of metal flanges 12 (only one of which is shown) which are welded to the mufiie adjacent the opposite ends thereof. The flanges 12 extend completely around the muffle and extend outwardly at right angles therefrom. A sheet metal shell 14 is suitably mounted on the outer ends of the flanges 12 so as to surround the muffle in spaced relation and complete the housing for the mufile. The oven is adapted to be mounted in an aperature in a wall or frame 16. To facilitate mounting of the oven, a plurality of studs are welded to the flanges or end plates 12. As shown in FIG. 1; the studs extend through a gasket 18 disposed between the end plates 12 and wall 16, and a plurality of nuts retain the parts in assembly. This manner of mounting the oven is, of course, only shown by wa of example, and other suitable means could be used.

The oven further comprises a pair of hinged doors 2t) and 22 which form end walls for the opposite ends of the mufiie while, at the same time, permitting access to the interior of the oven from either end of the mufiie. A vacuum pipe 24 extends from the bottom wall of the mutile through the shell 14 and is adapted to be connected to pumping means (not shown) for drawing a vacuum. A back fill pipe 26 also extends from the bottom wall of the mufile and through the shell 14. The back fill pipe is intended to be valved in order to provide means for breaking the vacuum in the oven and permit the pressure within the muffle to return to atmospheric pressure prior to opening the doors. A plurality of vertically arranged shelves 27 are provided within the mulile to accommodate parts to be treated within the oven. The shelves are fabricated from a ferrous metal, preferably stainless steel. The shelves are perforated to facilitate heat transfer vertically within the mufile and are preferably slidably mounted for ready removal from the muffle.

In order to provide a heat source for the even, a plurality of electrical resistance heaters 28 are arranged around the top, bottom and side walls of the mutlle. In the specific embodiment, these heaters are of a type gen erally referred to as tubular heaters. More specifically, as shown in FIG. 1, a plurality of such heaters are arranged in parallelspaced apart relation alongside each wall of the muffle. The heaters are mounted in a pair of overlying metal plates 30 and 32. The plates are provided with recesses conforming to the cross section of the heaters and the heaters are sandwiched between the plates with the walls of the recesses in the plates being in intimate contact with the heater sheathes. The heater and plate assemblies are bolted to the outer surfaces of the walls of the muflie with the bottom plates 32 in intimate contact with the mufile. The sandwiching of the heaters between the plates 30 and 32 facilitates mounting of the heaters on the muffle. Further, as can be seen from FIG. 1, each plate assembly is of relatively large planar area and covers the majority of the particular wall of the muffle. It is preferred that the plates have a relatively high thermal conductivity s0 that the heat provided by the heaters will be substantially uniformly distributed over the areas covered by the respective heater plate assembly. In this manner, the tendency of the heaters to provide hot spots within the muflle in the immedate area of the heaters will be substantially eliminated, and there will tend to be uniform heat distribution within the muffle.

As noted above, one of the problems in vacuum ovens of the type shown is that of providing optimum uniform heat distribution within the mufi'le. In order to provide more uniform heat distribution both vertically and horizontally Within the oven, the inner walls of the mufile, the end walls formed by the doors 2t), 22, the shelves 27 and the inside of the vacuum draw pipe 24 and back fill pipe 26 are Black Oxide coated. Thus, all of the surfaces exposed to or seeing the inside of the mufiie are Black Oxide coated. In accordance with the invention, the treatment provided serves to increase the resistance to oxidizing of the surfaces treated, while at the same time provides the inner surfaces of the mufile with a much higher thermal emissivity than would be provided by the usually highly reflective surface provided by stainless steel. The problem of heat distribution in an oven of the type described is complicated by the fact that the heat treatment is conducted under a partial vacuum, for example at pressures from 1X10- to 1X10 millimeters of mercury. At this low pressure, heat transfer within the mufile is substantially entirely by means of radiation rather than by convection. The Black Oxide coating provided in accordance with this invention will increase the radiation heat transfer from the mufile walls to improve the chiciency of the oven. Also, the particular treatment utilized provides a coloration and finish which is quite uniform so as to provide that the thermal emissivity of the inner surfaces of the mufiie will be uniform and there will be no reflective portions which could cause hot spots. The Black Oxide treatment specifically contemplated involves heating of the mufile at a relatively high temperature in an alkaline bath. Such treatments are normally of a proprietary nature. A specific one of such treatments contemplated is provided by the Rust Proofing and Metal Finishing Corporation of Cambridge, Massachusetts, under the name of Black Oxide No. 160. It will be understood that where the term Black Oxide is used herein, the term refers to a treatment such as that just identified.

As noted above, another problem in the operation of vacuum ovens is the rapid deterioration of the door seals because of the exposure of the seals to relatively high temperatures and low pressures. conditions tend to cause any volatiles in the seal material to boil off leaving the seal in a hard brittle condition which will seriously reduce its utility for the purpose intended. With reference to FIGS. 1 and 2, the door sealing arrangement is identical with respect to the doors 2t) and 22. Accordingly, in the interest of brevity, only the details of construction of the door 2% and its sealing arrangement have been shown and will be described. The door 2t) comprises a sheet metal shell 36 of rectangular cup-like configuration. A plate 38 is disposed within the door shell 36 and is fixed to the upstanding side walls of the door shell intermediate the end of the walls so that the plate 38 extends parallel to the general plane of the door in spaced relation generally midway between the ends of the side Walls of the door shell The door further includes a stainless steel pressure plate 4t) forming an inner wall for the door and an end wall for the mufile. The pressure plate 45) extends parallel to the general plane of the door and is mounted for movement toward and away from the plate 38. More specifically a plurality of pins 42 are mounted on the pressure plate and extend at right angles thereto and inwardly of the door. The pins 42 extend through apertures in the plate 38 with the bordering edge portions of the apertures supporting the pins for sliding movement. A plurality of coil springs 44 are arranged over the pins 42 and engaged at their opposite ends between the pressure plate 4t? and plate 33 for the purpose These environmental of urging the pressure plate away from the plate 38. The free ends of the pins 42 are provided with radially extending stops 46 to limit movement of pins, and thus the pressure plate, away from the plate 38. When the door is open, the springs 44 will urge the pressure plate outwardly of the door until the stops 46 engage the plate 38. When the door is closed, the pressure plate 40 engaged against a rectangular cross sectioned silicon rubber seal 48 arranged around the outside of the muffle. When the door is securely locked in the closed position, the seal 48 engages the pressure plate 40 and will cause the pressure plate 40 to be moved inwardly of the door against the force of the springs, whereby the pressure plate will be maintained in firm engagement with the seal 48.

The seal 48 extends entirely around the periphery of the mutile and, as can be seen in FIG. 2, the seal projects beyond the end of the mufile. In order to provide cooling for the seal 48, a cooling ring 50 is mounted on the end wall 12 of the oven housing. The cooling ring 50 is an elongated, rectangular cross sectional, hollow member extending completely around the seal 48 in contact with the side thereof opposite the muffle. A plurality of fittings 52 are provided on the cooling ring. These fittings extend through apertures in the end wall 12 of the oven housing and are adapted to be connected to a source of cooling liquid, such as water, as well as to a drain, whereby the cooling liquid may be continuously circulating through the cooling ring. As will be seen in FIG. 2, the wall of the cooling ring next adjacent the door is spaced from the pressure plate 40 when the door is closed. The fiat engaged surfaces of the seal and cooling ring assure good heat transfer between these members inasmuch as these surfaces conform to each other and the overlying surface of the ring is generally coextensive with the underlying surface of the seal. The cooling of the seal in the manner provided tends to alleviate the problem of the boiling off of the volatiles in the seal when the oven is operated at relatively high temperatures, for example 250 C. and relatively low pressures specified above. However, as noted above, the exposure of the seal to high temperature is only a portion of the problem; the other being the exposure of the seal to low pressures. Also, as noted above, the seal 48 projects beyond the end of the mufile so that, when the 'door is closed, the pressure plate 40 will be spaced from the end of the muflle. This assures that firm sealing will be provided between the pressure plate and silicon seal, and that there will be no metal to metal contact between the pressure plate and the muffle. However, the spacing of the ends of the mufile from the pressure plate is particularly important with regard to another aspect of the invention.

More specifically, the end of the mufile is spaced from the pressure plate a distance selected to provide what will be referred to as an optical seal between the interior of the mutfie and the silicon seal, whereby the lowest pressure to which the seal 4S will be exposed will be greater than the lowest pressure achieved within the interior of the mufiie. The term optical seal connotes a seal whereby the silicon seal 48 does not see the pressure within the mufile. In the embodiment of FIG. 2, the space 54 between the mufiie and pressure plate is approximately of an inch, which dimension is quite small as compared to the overall dimensions of the oven. In the specific embodiment, the oven is approximately 10 /2 inches in its cross sectional diameter and approximately 14 inches long. It will be apparent from FIG. 2 that the only portion of the seal directly exposed to the interior of the mufile is that portion which projects beyond the ends of the mufiie. When the oven is evacuated, the air at the outer end portions of the gap or space 54 can be drawn out only by passing through the very narrow passage provided by the space 54 and into the interior of the mufile. As the air is drawn from the motile, there will be no flow of air through the passage 5d which will readily sweep the molecules of air from the passage 54. This is particularly true as the pressure within the mutlle is substantially reduced. Rather, the molecules of air in the passage 54 will tend to move between the opposing surfaces of the end of the mufiie and pressure plate 40 substantially at right angles to these surfaces instead of moving parallel to these surfaces. This action materially retards the evacuation of the space 54 with the result that a higher pressure will be present within the passage 54 than will be present within the main body of the mufiie. This will be particularly true in the area of the passage immediately adjacent the seal 43. It can be seen that the molecules of air in the passage 54, and particularly at the end thereof adjacent the seal 48, follows what amounts to a tortuous path into the interior of the mullie. Admittedly, their path is not as tortuous as would be the case if the passage 54 were a labyrinth passage. However, the configuration shown has been found to be sufiicient for the purpose intended. It is to be understood, however, that a more complex optical seal is within the scope of this invention. Admittedly, there will be a tendency for the air molecules within the passage 54 to diffuse into the main body of the muffle until the pressure within the passage 54 is equal to the pressure within the motile. However, such diffusion will require a substantial length of time to occur. in practice, the oven will normally be opened every three or four hours, and in this short length of time, the advantages gained by the optical seal will not be lost to any substantial extent by diffusion.

Accordingly, it can be seen that by the door sealing arrangement of this invention the silicon rubber seal is both exposed to temperatures considerably less than that within the mufiie and is exposed to pressures substantially greater than the pressure within the mufiie. By this means, the service life of the seal has been materially improved. The specific arrangement of the cooling ring and rubber seal results in the cooling ring being disposed remotely from the muffle so as not to cause cooling of the ends of the mufiie with an attendant impairment of the uniformity of heating in this area of the mufile. Also, the cooling ring is not in contact with the pressure plate so as to preclude undesired cooling of the end wall of the muffle formed by the pressure plate.

Inasmuch as changes could be made in the above construction and apparently different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the language in the following claims is intended to cover all of the generic and specific features of the invention herein described and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

Having thus described our invention, we claim:

1. A vacuum oven having, in combination, a mufiie having an open end, a resilient, non-metallic seal member extending entirely around the outside of said end with a relatively small portion thereof projecting from said end, the mufile having support means for the seal member in position to resist compression thereof toward said end, a liquid cooling ring surrounding the seal member in heat transfer contact therewith and separated from the mutlle by the seal member, the seal member also extending beyond said open end and the cooling ring, and a door for the mufiie having a pressure plate to engage the seal member, a support to carry the pressure plate and to close about the end of the mulfle, and spring suspension means engaged between said support and pres sure plate, whereby the plate compresses the seal memher until the projection of the seal member beyond the end of the muflie is a very small value, said value depending upon a predetermined deflection of said spring suspension means produced by closure of said support about said end of the mutlie.

2. The combination according to claim 1 wherein the seal member is composed of silicon rubber.

3. The combination according to claim 1 in which the spring suspension means include a plurality of guide members supporting the pressure plate on the support for movement toward and away from the seal member and a plurality of compression springs urging the pressure plate from the support toward the seal member.

4. The combination according to claim 1 in which the support for the pressure plate is hinged to the end of the muffle.

References Qited in the file of this patent UNITED STATES PATENTS 622,546 Shaw Apr. 4, 1899 814,726 Potter Mar. 13, 1906 871,421 Koppers Nov. 19, 1907 1,425,679 Nicholson Aug. 15, 1922 1,540,401 Kelly et al. June 2, 1925 2,964,307 Van Dine Dec. 13, 1960 3,002,735 Baker et al. Oct. 3, 1961 3,020,647 Butts Feb. 13, 1962 3,058,232 Farliass Oct. 16, 1962 FOREIGN PATENTS 794,672 Great Britain May 7, 1938 

1. A VACUUM OVEN HAVING, IN COMBINATION, A MUFFLE HAVING AN OPEN END, A RESILIENT, NON-METALLIC SEAL MEMBER EXTENDING ENTIRELY AROUND THE OUTSIDE OF SAID END WITH A RELATIVELY SMALL PORTION THEREOF PROJECTING FROM SAID END, THE MUFFLE HAVING SUPPORT MEANS FOR THE SEAL MEMBER IN POSITION TO RESIST COMPRESSION THEREOF TOWARD SAID END, A LIQUID COOLING RING SURROUNDING THE SEAL MEMBER IN HEAT TRANSFER CONTACT THEREWITH AND SEPARATED FROM THE MUFFLE BY THE SEAL MEMBER, THE SEAL MEMBER ALSO EXTENDING BEYOND SAID OPEN END AND THE COOLING RING, AND A DOOR FOR THE MUFFLE HAVING A PRESSURE PLATE TO ENGAGE THE SEAL MEMBER, A SUPPORT TO CARRY THE PRESSURE PLATE AND TO CLOST ABOUT THE END OF THE MUFFLE, AND SPRING SUSPENSION MEANS ENGAGED BETWEEN SAID SUPPORT AND PRESSURE PLATE, WHEREBY THE PLATE COMPRESSES THE SEAL MEM- 